In a wireless communication environment, it is often the case that a transmitter must be operated at relatively low power. Such a constraint may be a function of limited power being available from the transmitter's power supply, such as for battery operated mobile units. In addition, the use of low transmitter output power may be required to limit interference with other signals in the same geographic proximity. While such low power operation may provide various benefits, it also has the negative consequence of significantly limiting the range at which the signal from the transmitter can be reliably received. In the art, that diminished range for a low power transmitter is ordinarily accommodated by locating the receiver(s) for the signal from such a transmitter within the limits of that diminished transmission range. For example, in a wireless communication arrangement where a number of mobile units are served by a base station, the boundary of the service area for that base station will be constrained to the range of the low-power transmitters at the mobile stations.
An alternative approach may be used in the case of a number of low-power transmitter units being positioned in a large distributed array, where each transmitter is required to periodically communicate with a central receiver. In this circumstance, the central receiver can be located proximate to one of the low-power transmitters. With such an arrangement, an outlying (i.e., distant from the transmitter located proximate to the receiver) low-power transmitter having information to send to the central receiver transmits that information to a nearby unit in the array. (It should be understood that the low-power transmitter units in the array will include an associated receiver for such inter unit communications, as well as to receive instruction from the central receiver location). If that nearby unit in the array happens to be the unit directly linked to the central receiver, it then retransmits the information from the outlying unit to the central receiver. Otherwise, the receiving nearby unit retransmits the information on to a unit in the array nearby to it, and in the direction of the unit that is directly linked to the central receiver. Such transmission and retransmission continues through multiple hops until the information reaches the unit directly linked to the central receiver, at which point it is transmitted to the central receiver. For instructions from the central receiver location to one or more of the low-power transmitter units, that process may be carried out in reverse (in the case where the transmitter associated with the central receiver is also constrained to such low-power operation).
Another technique, called Bell Labs Layered Space Time, and usually referred to by the acronym “BLAST,” can also be used for increasing the useful range of low-power transmitters (or, alternately, increasing the transmission rate for such transmitters). The BLAST technique equips both the transmitter and the receiver with multiple antenna arrays and distributes the data stream for the transmitter among the multiple antennas making up the transmitter antenna array. In this arrangement, sophisticated algorithms developed for BLAST are applied at the receiver (using complex signal processing equipment to implement those algorithms) to analyze the information in the multiple transmission streams received at the receiver. Through such algorithmic processing, the BLAST approach achieves a significant improvement in throughput across the transmission channel relative to other low-power methods. Since throughput in a transmission channel is substantially a function of the transmission energy per bit seen at the receiver, the higher energy per bit provided by the BLAST technique could also provide a given throughput at a greater range than could have been achieved with classic wireless transmission systems.
Nonetheless, while BLAST can be used to improve the transmission range for a low-power transmitter, that improvement occurs at a significant cost in the required additional algorithmic analysis, and the digital signal processing components needed to carry out that analysis.